Complete hydatidiform moles, an abnormal pregnancy of androgenetic origin consisting of swollen villi without an embryo, were studied to determine the protein constitution of vesicular fluids in the swollen villi and the ability of the molar tissue to synthesize protein. All 16 serum proteins tested were identified in the molar fluid. A reverse correlation was obtained when the logarithms of the molar fluid versus maternal serum concentration ratios of different proteins were plotted against the logarithms of their molecular weights. Logarithmic regression analysis showed a linear regression with a high coefficient (gamma = 0.9626). Phenotyping of haptoglobin in the molar fluid and in the parental sera indicated a definite contribution of maternal proteins to the molar fluid. The molar tissues synthesized no serum proteins but produced human chorionic gonadotrophin beta-subunit (hCG-beta) and pregnancy-specific beta 1-glycoprotein (SP1) when examined in tissue cultures metabolically labelled with [35S]methionine. These findings indicate that a variety of maternal serum protein components is transferred to the molar fluid without being selected by the molar trophoblastic cells.